Cutting air pollution will start preventing illness almost straight away, according to a study which looked at death rates in Hong Kong.

In 1990, the then UK colony introduced regulations to reduce sulphur emissions from cars.

Researchers from the Universities of London and Hong Kong tried to gauge the impact of this.

The Hong Kong intervention provides direct evidence that control of this pollution has immediate - and long-term - health benefits

Professor Anthony Hedley, University of Hong Kong

Their research, published in The Lancet medical journal this week, found that, on average, every resident of Hong Kong gained weeks of extra life expectancy for every year they breathed the cleaner air.

Women gained just over 20 days on average per year - and men 41 days.

It is now estimated that Hong Kong residents will live on average several months longer because they live in a less polluted city.

Deaths from respiratory disease fell by 5% each year from the introduction of the measure, and heart disease by 2% each year.

Anthony Hedley, a professor of community medicine at the University of Hong Kong, said: "Pollution from sulphur-rich fuels has an effect on death rates, especially respiratory and cardiovascular deaths.

"The outcome of the Hong Kong intervention provides direct evidence that control of this pollution has immediate - and long-term - health benefits."

Heavily polluted

Hong Kong was handed over to Chinese sovereignty in 1997, while remaining an autonomous region.

It was notorious for being one of the world's most heavily polluted cities - and the coastal waters surrounding it are still said to be badly tainted.

Its problems with air pollution in general have yet to be solved - on September 12 this year, levels of particulates from vehicle exhausts were so bad that the population was warned to stay indoors.

A report in the South China Morning Post suggested that particulate pollution sometimes reached more than four times US safety limits.

A previous study estimated that annual deaths as a result of respiratory or cardiovascular illness topped 6,000.

However, regulations now require all power plants and vehicles to use fuel with a reduced sulphur content.